So I have a red afro wig. It's nothing fancy, just one of those crappy costume wigs from a retail store, but I need to make half of it white. I know I can't bleach it, but I was wondering if there was a way to paint it? Would spray paint work? or maybe that spray on dye? What would be the best way to go about doing it without ruining it?

You can't dye something white. You can only change the tint of a color or make it darker.

Spray paint will make it disgusting. I've used it on wigs. The wig will become crunchy, stiff as hell, and if you do it heavily enough to get the surface solid white, it won't look much like wig fiber anymore. If any of the wig fiber is near your eyes, it will poke you in the eyes and hurt. You might also shed white paint flakes.

You'd do better to find a white afro wig, cut both of them in half, and frankenstein the different colors together.